1. Field Of The Invention
The invention relates to a device for preparing hot water provided with a pressure pump with a supply pipe for supplying relatively cold water to the pressure pump and an outlet connected to a heating pipe, which is in heat-exchanging contact with a heating body, with an outlet valve in the heating pipe and a coffee holder, connected to the heating pipe, for accommodating a bed of coffee, whereby the heating body comprises a reservoir that is openly connected to the environment, with a heating element and water in the reservoir as a heat-exchange medium.
2. Description Of The Related Art
Such a device is known, for example, in an espresso machine that is described in U.S. Pat. No. 5,498,757. In such a unit, coffee extract is obtained by forcing non-boiling water at a temperature of approximately 85° C. to 105° C. through a bed of ground coffee at a pressure of approximately 10 bars as a result of which the required flavours and aromatic substances end up in the water. In this way between 25 ml and 130 ml of coffee extract are produced with approximately 15 g of ground coffee.
When preparing coffee, a specially designed appliance is used that more or less carries out the coffee-making procedure automatically. In most of these appliances cold water is pressurised with a pump in order to then be passed through a heating element where it is heated to the required temperature. The required pressure is achieved by pressing the bed of ground coffee into a more or less compact mass which restricts the water from flowing through it, thereby achieving a build up of pressure.
In order to reduce the pressure after the coffee-making process the flow of water is taken to an open outlet after which the remaining water is generally directed to a drain.
Draining the water still remaining in the system also prevents this remaining water boiling as a result of overheating and producing steam, or the expansion of the still present cold water, which in both cases could lead to unacceptable increase in pressure as a result of which damage to the system could occur. The disadvantage of draining the hot water is, however, that the known system is not efficient in terms of hot water use. Furthermore, a special drain has to be applied which connects either to a central water drain, or has to collect the water in a reservoir which must then be regularly emptied.
As the known heating pipe is in heat-exchanging contact with the water in the reservoir that is open to the atmosphere, for example by the heating pipe passing through the reservoir, the temperature of the water in the heating pipe can never be higher than the boiling point of water. In this way undesirable steam formation and pressure build up in the heating pipe are prevented.
When making coffee the pressure pump should force pressurised water through the heating pipeline to the bed of coffee. By correctly dimensioning the heating pipeline the correct temperature is reached. On closing the dispensing valve the pressure in the pipe section downstream of the dispensing valve is equivalent to the atmospheric pressure.